Humidity and odor control is an important factor related to both human comfort and health. A relative humidity ranging from 20% to 80% is permissible for most activity; however, there is now a belief that a more optimum range of about 30% to about 60% is preferable to minimize and/or eliminate bacterial, viral and fungal growth. Maintaining this appropriate level of humidity as well as odor control in passenger compartments of transportation vehicles presents a particular problem. Existing dehumidifying devices either remove too much moisture from the air or do little to remediate odors.
Most odors in passenger compartments of transportation vehicles are the result of volatile organic compounds, VOCs, present in the circulating air. VOCs are formed in large quantities but at relatively low concentrations from engine exhaust, solvents, gas turbines, cogeneration plants, petrochemical plants, and in many industrial processes where waste gases contain such materials as vapors of various solvents, inks, paints, and so forth. VOCs contain not only hydrocarbons--saturated, unsaturated, and aromatic--but also contain oxygenated materials such as alcohols, esters, ethers, and acids, nitrogen containing compounds (principally amines), sulfur containing materials (mercaptans and thioethers) and halogen-containing materials, especially chlorine-substituted hydrocarbons but also organic fluorides and bromides. The presence of these VOCs in the gas stream can present a health risk or cause the gas stream to have an unpleasant odor.
Conventional adsorption systems for solvent recovery from humid air typically are operated until the solvent concentration in the outlet gas stream reaches a detectable preset breakthrough level whereupon the gas flow to the adsorber is stopped. The adsorbent bed then contains solvent, other condensable organic contaminants, and some amount of water which depends on the inlet relative humidity of the solvent laden gas stream. At this point, present-day techniques involve the introduction of a hot inert gas or steam, either saturated or superheated, which displaces the solvent from the adsorbent to produce a solvent/water mixture upon condensation. Typically two adsorber beds are used, where one is adsorbing while the other bed undergoes regeneration. More recent technology for regenerating and recovering solvent from adsorbent beds involves the use of inert gases (though for some solvents, air also can be used) and low temperature condensation of the solvent from the regenerating gas. Neither of these techniques is directly applicable for use in conditioning passenger compartments of transportation vehicles.
The removal of volatile organic compounds (VOC) from air by adsorption is most often accomplished by thermal swing adsorption (TSA). Air streams needing treatment can be found in most chemical and manufacturing plants, especially those using solvents. At concentration levels from 500 to 15,000 ppm, recovery of the VOC from steam used to thermally regenerate activated carbon adsorbent is economically justified. Concentrations above 15,000 ppm are typically in the explosive range and require the use of a hot inert gas rather than air for regeneration. Below about 500 ppm, recovery is not economically justifiable, but environmental concerns often dictate adsorptive recovery followed by destruction. In passenger cabins, the level of VOCs is relatively low, typically ranging from 0.01 to about 10 ppm. At this level, the VOCs can be safely returned to the environment. Activated carbon is the traditional adsorbent for these applications, which represents the second largest use for gas phase carbons. U.S. Pat. No. 4,421,532 discloses a process for the recovery of VOCs from industrial waste gases by thermal swing adsorption including the use of hot inert gases circulating in a closed cycle to desorb the VOCs.
One device employed for drying air in confined areas such as a home, a ship or a building down to 0.001 lbs water per pound of air is exemplified in U.S. Pat. No. 4,134,743 to Macriss et al. Macriss et al. disclose a process and apparatus wherein the adsorbent body is a wheel of thin sheets or layers of fibrous material containing about 10 to 90% by weight of a freely divided molecular sieve material. The apparatus includes a means for passing air to be processed in one direction through the wheel and a means for passing a regenerative air stream countercurrent to the air to be processed. In addition a cooling stream is provided in a direction cocurrent with the air stream.
U.S. Pat. No. 4,887,438 to Meckler discloses a desiccant assisted air conditioning system for delivering dehumidified refrigerated air to a conditioned space based on an adsorbent wheel. Meckler teaches the use of a desiccant wheel coated with silica gel, or a preferred hygroscopic salt, lithium chloride, to remove moisture from air. Meckler uses the waste heat from the refrigeration condenser to heat the reactivation air and employs a liquid refrigerant injection into the compressor to increase the pressure ratio in a positive displacement compressor to counter the problem of "thermal dumpback". Thermal dumpback is the associated heat conducted from the desiccant wheel to the treated air which occurs following the exposure of the wheel to heated regeneration air. This associated heat adds to the overall cooling load on the refrigeration system.
A U.S. Pat. No. 5,242,473 to Cgasahara discloses a gas dehumidifying apparatus which exhibits improved dehumidification efficiency to provide a treated gas with a high dryness level employing two dehumidifier rotors, wherein the second rotor uses a synthetic zeolite. Gas to be dehumidified is passed first to a silica gel coated rotor and then to the zeolite coated rotor. The rotors are regenerated by supplying a stream of heated gas through the second rotor and then the first rotor so that the adsorbent in the first rotor is regenerated at a lower temperature than the zeolite in the second rotor. A portion of the treated gas is used to countercurrently cool the rotors following regeneration.
Of those techniques for removing VOCs in low concentrations from a gas stream by adsorption, the most common method is exemplified in U.S. Pat. No. 4,402,717 to Izumo et al. In Izumo et al., an apparatus for removing moisture and odors from a gas stream comprises a cylindrical honeycomb structure made from corrugated paper, uniformly coated with an adsorbent and formed in the shape of a disk or wheel. The multiplicity of adsorbent-coated parallel flow passages formed by the corrugations in the paper serve as gas passage ways which are separated as a zone for the removal of water and odor causing components in the gas, and as a zone for the regeneration of the adsorbent. The zones for removal and regeneration are continuously shiftable as the wheel is rotated circumferentially about its centerline. Labyrinth seals separate the outer side of the rotary structure from the cylindrical wall of a sealed casing.
The coated wheel or rotor units of the prior art suffer a number of disadvantages. Generally, the wheels are quite large, with wheels typically ranging from 2 to 14 feet in diameter. One well-known problem is the hereinabove mentioned thermal dumpback problem. Some adsorbents require desorption to take place at temperatures which are significantly higher than adsorption temperatures. The greater the difference between the adsorption temperature and the desorption temperature, the greater the heat load on any air conditioning system resulting from the thermal dumpback, or sensible heat load placed on the adsorbent wheel. Configurations are sought to minimize this effect within the adsorption apparatus.
A second problem is related to the adsorbent employed. Hydrophilic adsorbents such as silica gel are typically chosen for dehumidification applications, but hydrophilic adsorbents are poor adsorbents for VOC removal. One such process combination is exemplified in U.S. Pat. No. 5,181,942. On the other hand, hydrophobic adsorbents, such as high silica zeolites are typically recommended for VOC removal applications, yet are poor adsorbents for dehumidification application. Thus, applications for both dehumidification and VOC removal might typically require both types of adsorbents. Processes are sought wherein a single adsorbent can be employed for both operations. Furthermore, the adsorbent-coated paper and some adsorbent salts have a limited range of humidity and temperature within which they can maintain structural integrity. This failure also limits the regeneration medium to dry, moderate temperature gases and air. The contact between the adsorbent and the gas stream and hence the adsorbent capacity for the VOCs is limited to very thin layers of adsorbent on the surface of the fiber. This feature also limits the ultimate life of the adsorbent wheel, resulting in frequent wheel replacement.
In a report prepared for the Gas Research Institute by R. K. Collier, Jr., titled, "Advanced Desiccant Materials Assessment, Phase II" (GRI 88-0125), published January 1988, pages 38-46, Collier identified the critical parameters of desiccant cooling system as being dependent upon the physical and thermodynamic properties of the adsorbents employed in the desiccant cooling system. Collier characterized the most desirable desiccant or adsorbent as having a "Type 1M" adsorption isotherm. The Type 1M isotherm has a continuously negative curvature which provides a good driving force for adsorption at low partial pressures (relative humidity) and a heat of adsorption which is slightly greater than the heat of vaporization/condensation of water. Became these properties are generally not found together in the same adsorbent, and these properties are typically mutually exclusive in available commercial adsorbents, Collier theorized that, if such a material could be developed, its use would have an enormous impact on the performance of the resulting sorption cooling system.
An adsorber module is sought which does not have the adsorbent limitations of prior art adsorbent wheels, has an increased adsorbent capacity for the removal of VOCs and water, and has a lower sensitivity to the effects of sensible heat buildup or thermal dumping for use in dehumidifying and removing VOCs from passenger compartments of transportation vehicles.